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1ashd(7)
2=======
3
4NAME
5----
6ashd - A Sane HTTP Daemon
7
8DESCRIPTION
9-----------
10
11This document describes the architecture and protocol of ashd
12technically. If you want a brief overview, please see the homepage at
13<http://www.dolda2000.com/~fredrik/ashd/>.
14
15The basic premise of ashd is that of standard Unix philosophy; it
16consists of a number of different programs, each specialized to one
17precise task, passing HTTP requests around to each other in a manner
18akin to standard Unix pipelines. This document describes the protocols
19and conventions used between such programs that allows them to
20interoperate.
21
22REQUESTS
23--------
24
25All requests within ashd are created by *htparser*(1), which speaks
26HTTP with its clients, translates the requests it receives into ashd
27format, and passes them to a specified handler program. The handler
28program may choose to respond to the request itself, or pass it on to
29another handler for further processing.
30
31A request in ashd format consists of 4 structural parts:
32
33HTTP method, URL and version::
34
35 The HTTP header line information, exactly as specified by the
36 client. That is, any escape sequences in the URL are passed
37 around in non-processed form, since each program needs to
38 handle escape processing in its own way.
39
40The rest string::
41
42 The 'rest string' (sometimes referred to as the 'point', in
43 deference to Emacs parlance) is the part of the URL which
44 remains to be processed. Each handler program is free to
45 modify the rest string (usually, but not necessarily, by
46 removing leading parts of it) before passing the request on to
47 another handler. When *htparser*(1) initially constructs a
48 request, it forms the rest string from the URL by stripping
49 off the initial slash and the query parameters. In other
50 words, a request to `/a/b/c?d=e` will be given the initial
51 rest string `a/b/c`.
52
53The HTTP headers::
54
55 The HTTP headers are parsed and passed along with the request
56 as they are, but *htparser*(1) itself, and some handler
57 programs, add some more headers, prefixed with `X-Ash-`,
58 and to safeguard any potentially sensitive such headers from
59 forgery, *htparser*(1) strips any headers with that prefix
60 from the incoming request.
61
62The response socket::
63
64 Along with the request information, a socket for responding is
65 passed. A handler program that wishes to actually respond to a
66 request needs only output a well-formed HTTP response on this
67 socket and then close it. The details are covered below, but
68 note that the socket is connected to *htparser*(1) rather than
69 the client itself, and that *htparser* will do any transfer
70 encoding that may be required for HTTP keep-alive. The
71 response socket is also used for reading the request-body, if
72 the client provides one.
73
74HANDLERS
75--------
76
77Handler programs are started either by *htparser*(1) itself, or in
78turn by other handler programs, and certain conventions are observed
79in that process.
80
81There are two basic types of handler programs, persistent and
82transient, which determines the convention used in starting them. A
83persistent program will continue running indefinitely, and handle any
84amount of requests during its lifetime, while a transient program will
85handle one request only and then exit. The convention of transient
86programs was created mainly for convenience, since it is easier to
87write such programs. The *htparser*(1) program will only start a
88persistent program as the root handler.
89
90A persistent handler program, when started, is passed a Unix socket of
91SEQPACKET type on standard input. Its parent program will then pass
92one datagram per request on that socket, containing the above listed
93parts of the request using the datagram format described below. By
94convention, the handler program should exit normally if it receives
95end-of-file on the socket.
96
97A transient program, when started, has the response socket connected
98to its standard input and output (standard error is inherited from the
99parent process). It may be provided arbitrary arguments as supplied by
100the program starting it, but the last three arguments are the HTTP
101method, the raw URL and the rest string, in that order. The HTTP
102headers are converted into environment variables by turning them into
103uppercase, replacing any dashs with underscores, and prefixing them
104with `REQ_`. For example, the HTTP `Host` header will be passed as
105`REQ_HOST`. The HTTP protocol version is passed in an environment
106variable called `HTTP_VERSION`. It is passed in full; i.e. as
107`HTTP/1.1`, rather than just `1.1`.
108
109The response socket, as mentioned above, is also used for reading the
110request-body if the client provides one. For such purposes,
111*htparser*(1) ensures that the reader sees end-of-file at the end of
112the request-body, so that the reader (unlike in, for example, CGI)
113does not have to worry about the Content-Length header and counting
114bytes when reading.
115
116To respond, the handler program needs to write an ordinary HTTP
117response to the response socket. That is, one line containing the HTTP
118version, status code and status text, followed by any number of lines
119with headers, followed by an empty line, followed by the
120response-body. Basic familiarity with HTTP should relieve this
121document of detailing the exact format of such a response, but the
122following points are noteworthy:
123
124 * The HTTP version is actually ignored; it must simply be there for
125 completeness.
126
127 * In the header, Unix line endings are accepted; *htparser*(1) will
128 still use CRLF line endings when passing the response to the
129 client.
130
131 * The response socket should be closed when the entire body has been
132 written. *htparser*(1) itself will take care of anything needed for
133 HTTP keep-alive, such as chunking. It is recommended, however, that
134 the handler program provides the Content-Length header if it can be
135 calculated in advance, since *htparser*(1) will not need to add
136 chunking in such cases.
137
138 * *htparser*(1) will not provide an error message to the client in
139 case the response socket is closed before a complete response has
140 been written to it, so a handler program should always provide an
141 error message by itself if a request cannot be handled for some
142 reason.
143
144PROTOCOL
145--------
146
147The datagram format used for persistent handler programs is simply a
148sequence of NUL-terminated strings. The datagram starts with the HTTP
149method, the URL, the HTTP version and the rest string, in that
150order. They are followed by an arbitrary number of string pairs, one
151for each header; the first string in a pair being the header name, and
152the second being the value. The headers are terminated by one instance
153of the empty string.
154
155Along with the datagram, the response socket is passed using the
156SCM_RIGHTS ancillary message for Unix sockets. See *unix*(7),
157*recvmsg*(2) and *sendmsg*(2) for more information. Each datagram will
158have exactly one associated socket passed with it.
159
160AUTHOR
161------
162Fredrik Tolf <fredrik@dolda2000.com>
163
164SEE ALSO
165--------
166*htparser*(1), RFC 2616